One proposed optical scanning observation apparatus is an optical scanning endoscope that holds the tip of an optical fiber for illumination to allow oscillation, scans an object of observation with illumination light by vibrating the tip, and generates an image by detecting light that is reflected, scattered, or the like, or light generated on the object of observation, such as fluorescent light (for example, see JP 5190267 B2 (PTL 1)). In such an apparatus, the object of observation is scanned with illumination light in a spiral shape (spiral scan) to acquire an image.